This invention relates to injection molding, and more particularly to an improved electrical heater designed for use with injection molders.
Electrical heaters are well known in the injection molding industry for the purpose of directly or indirectly heating plastic resins to the point of melting. A heater of this type, is shown in U.S. Pat. No. 4,268,240 to Rees et al. This heater exemplifies a typical electrical nozzle heater which has an inside diameter substantially equal to the outside diameter of the nozzle. The heater is adapted to be slipped over the nozzle and then tightened into place using any effective fastening means. If the heater should become defective or if it should short circuit, the task of removing and replacing the heater can be a laborious task because of the associated wiring. If there are many heaters in the system, all having wires extending in the same pathway through an electrical junction box or the like, the task of removing and replacing the wires becomes even more complex.
Another problem relating to the use of heaters in injection molding systems, arises when a single power source is used to power all the heaters. Typically, the single power source will be fused but the fuse will be capable of permitting full power to all of the heaters comprising the system, simultaneously. If one heater should short out, the electrical trip circuit of the power source may not be sufficient to immediately blow the fuse, especially if the other heaters are operating at a low current draw. This can result in overheating of the electrically weakest portion of the electrical circuit. As a result, in some cases, an electrical connection pin may burn up or the wiring itself may melt. Both of these situations may result in damage to other wiring or components of the system, causing increased cost based on component repairs, part replacement and production stoppage.
Other problems exist with using currently available electrical heater systems for injection molders. For example, if incorrect wiring is accidentally installed with the electrical heaters, an excessive supply of power may destroy the heaters before a fault is detected and rectified by a power source having one main fuse. In addition, it may be found that the electrical power circuit is operating in an environment within a mold, that is much higher in temperature than anticipated in the original design of the electrical circuit. Accordingly, such higher temperatures can lead to failure of the circuit and damage thereto. As an added problem, electrical safety hazards can occur.
U.S. Pat. No. 5,030,084 to Gellert et al. addresses wiring assembly problems in injection molders by providing a pre-wired conduit system which eases integration of the heater conduit electrical systems into the hot runner. This approach does permit initial benefits, for example, often cumbersome and complicated wiring can be performed at the manufacturers location by experienced personnel prior to shipment to the customer rather than at the molder's site. However, because of the design, repairing a subsequent electrical problem such as replacing a heater and associated wires may still require significant disassembly of the hot runner system. As a result, labor may be intensified rather than reduced. Also, the system discloses no additional means for protecting against overcurrent damage to the electrical circuits or heaters, should a short or surge be encountered.
There is, therefore, a need in the injection molding art for an electrical heater system having means associated with each heater comprising the system for protecting the heaters against overcurrent damage and which allows easy repair.